Highly Efficient, Organocatalytic Aerobic Alcohol Oxidation

Shibuya, Masabumi; Osada, Yuji; Sasano, Yusuke; Tomizawa, Masaki; Iwabuchi, Yoshiharu

doi:10.1021/ja110940c

Cited by 184 publications

(88 citation statements)

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“…The absence of hydrogen atoms at the ␣-carbon atom in sterically hindered N-oxy radicals such as TEMPO prevents their degradation via elimination reactions. More recently, attention has been drawn to the use of sterically unhindered nitroxyl radicals such as AZADO [75,76] and ABNO [77,78], and derivatives thereof (Fig. 8), which possess ␣-hydrogens but are nonetheless stable toward elimination because the resulting double bond would be formed at a bridgehead and, hence, violate Bredt's rule [79].…”

Section: Stable N-oxy Radicals As Organocatalystsmentioning

confidence: 99%

Recent advances in green catalytic oxidations of alcohols in aqueous media

2015

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Section: Stable N-oxy Radicals As Organocatalystsmentioning

confidence: 99%

Recent advances in green catalytic oxidations of alcohols in aqueous media

2015

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“…[11][12][13][14][15] From the combination of TEMPO with iodine, the next reactive species were considered to generate intermediates 3 or 4 (two different mechanisms) in Fig 3. In addition, the typical regeneration of TEMPO from TEMPO-H was not considered in the ionic mechanism in …”

Section: Figmentioning

confidence: 99%

Metal-Free α-C(sp<sup>3</sup>)–H Functionalized Oxidative Cyclization of Tertiary <em>N,N</em>-Diaryl Amino Alcohols: Theoretical Approach for Mechanistic Pathway

Ullah¹,

Kim²

2017

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Abstract:The mechanistic pathway of TEMPO/I2-mediated oxidative cyclization of N,N-diaryl amino alcohols 1 is investigated in this study. Based on our direct empirical experiments, three key intermediates (the aminium radical cation 3, the α-aminoalkyl radical 4, and the iminium 5), four kind reactive species (radical TEMPO, cationic TEMPO, TEMPO-I and iodo radical) and three kind pathways (1. SET/PCET mechanism, 2. HAT/1,6-H transfer mechanism, 3. Ionic mechanism) were assumed. Under the assumption, nine free energy diagrams are acquired through DFT calculation. From the comparison of the solution phase free energy, some possibility can be excluded and then the chosen plausible mechanisms are concretized with the more stable intermediate 7.

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“…In all the cases, the reaction completed within several hours with Nor-AZADO, of which efficiency is comparable to that of 5-F-AZADO which exhibit the highest activity for aerobic oxidation in AZADO derivatives. 29) In fact, TEMPO failed to serve as an efficient catalyst in all the cases. Although the structural difference between Nor-AZADO and AZADO is only one methylene group in their skeletons, they exhibit different catalytic activities evidently.…”

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confidence: 96%

“…We next examined the aerobic oxidation conditions [6][7][8][14][15][16][17][18][19] using the nitroxyl radical/NaNO 2 /Air/AcOH system, 29) which we have recently reported as versatile transition-metal-free and halogen-free conditions.…”

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confidence: 99%

“…Note that the hydrobenzoin (28), which is prone to form benzaldehyde because of its easy carbon-carbon bond cleavage in some other oxidation systems, 30) was oxidized to benzil (29) in quantitative yield without oxidative cleavage (entry 14). This system is also effective for the 3-quinuclidinol (32), which contains an amine functionality, although a 10 mol% catalyst load is needed (entry 16).…”

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9-Azanoradamantane N-Oxyl (Nor-AZADO): A Highly Active Organocatalyst for Alcohol Oxidation

et al. 2011

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A highly active organocatalyst for alcohol oxidation has been developed. 9-Azanoradamantane N-oxyl (Nor-AZADO 4), constituting an unhindered, stable nitroxyl radical, exhibits superior catalytic activity to 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and AZADOs in the oxidation of alcohols to their corresponding carbonyl compounds.Key words organocatalyst; nitroxyl radical; alcohol oxidation; aerobic oxidation; 2-azaadamantane N-oxyl AZADO The oxidation of alcohols to their corresponding carbonyl compounds is one of the most fundamental transformations in both laboratory synthesis and industrial manufacturing, and numerous methods of such oxidation have been developed.1,2) In recent years, a stable class of nitroxyl radicals, as exemplified by TEMPO [2,2,6,6-tetramethyl piperidinyl 1-oxyl (1); Fig. 1], has been extensively used as a catalyst for the oxidation of alcohols in a wide range of chemistry. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] In particular, TEMPO-catalyzed alcohol oxidation has high priority in the pharmaceutical industry as an efficient, mild, and environmentally acceptable method. However, TEMPO is often inefficient in the oxidation of structurally hindered alcohols, posing a problem in the oxidation of secondary alcohols.We have recently reported that a less-hindered class of nitroxyl radicals, namely, 2-azaadamantane N-oxyls [AZADOs; AZADO (2a) and 1-Me-AZADO (2b)] and 9-azabicyclo[3.3.1]nonane N-oxyl [ABNO (3)], exhibit significantly enhanced reactivity compared with TEMPO.20-26) They exhibit extremely high activities toward a wide range of alcohols, including structurally hindered secondary alcohols that TEMPO fails to efficiently oxidize. 20,21) A structure-activity comparison indicated that the steric congestion near the active center exerts a critical impact on catalytic efficiency. 20) In our continuous interest in nitroxyl-radical-based oxidation, we envisaged that the catalytic activity of nitroxyl radicals can further be enhanced by reducing the steric factors around their active center. We herein report the advanced catalytic activity of 9-azanoradamantane N-oxyl [Nor-AZADO (4); Fig. 1]. This new catalyst exhibits superior catalytic efficiency to AZADO, 1-Me-AZADO and ABNO as well as to TEMPO.Nor-AZADO (4) was firstly synthesized in 1978 by Dupeyre and Rassat and reported to be a stable class of nitroxyl radicals.27) To the best of our knowledge, however, no study of Nor-AZADO has been reported since then. We were particularly interested in determining whether Nor-AZADO, which is a constraint variant of AZADO, produces the oxoammonium ions, which are useful for alcohol oxidation.Nor-AZADO was prepared in five steps from acetonedicarboxylic acid (5), glutaraldehyde, and benzylamine, by a previous method (Chart 1) with slight modification.

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Highly Efficient, Organocatalytic Aerobic Alcohol Oxidation

Cited by 184 publications

References 37 publications

Recent advances in green catalytic oxidations of alcohols in aqueous media

Recent advances in green catalytic oxidations of alcohols in aqueous media

Metal-Free α-C(sp<sup>3</sup>)–H Functionalized Oxidative Cyclization of Tertiary <em>N,N</em>-Diaryl Amino Alcohols: Theoretical Approach for Mechanistic Pathway

9-Azanoradamantane N-Oxyl (Nor-AZADO): A Highly Active Organocatalyst for Alcohol Oxidation

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Highly Efficient, Organocatalytic Aerobic Alcohol Oxidation

Cited by 184 publications

References 37 publications

Recent advances in green catalytic oxidations of alcohols in aqueous media

Recent advances in green catalytic oxidations of alcohols in aqueous media

Metal-Free &alpha;-C(sp<sup>3</sup>)&ndash;H Functionalized Oxidative Cyclization of Tertiary <em>N,N</em>-Diaryl Amino Alcohols: Theoretical Approach for Mechanistic Pathway

9-Azanoradamantane N-Oxyl (Nor-AZADO): A Highly Active Organocatalyst for Alcohol Oxidation

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Metal-Free α-C(sp<sup>3</sup>)–H Functionalized Oxidative Cyclization of Tertiary <em>N,N</em>-Diaryl Amino Alcohols: Theoretical Approach for Mechanistic Pathway